Juveniles of artificially bred Tachypleus tridentatus were reared in the laboratory for 15.5 mo. Fast growth (at least 3 times faster than in previous studies) and low mortality were observed, with juveniles reaching the 9th instar stage and having a cumulative mortality of 61.3% at the end of the rearing period. Such enhanced growth can potentially be attributed to high seawater temperature, sufficient living space and constant water flow. Isometric growth of prosomal width and other growth parameters including opisthosoma width, eye distance and prosomal length was found. Two distinct growth phases were detected for the opisthosoma length, with zero growth from the 1st to 2nd instar but isometric growth from the 2nd to 9th instar stage. Growth in telson length was positively allometric from the 1st to 3rd instar and isometric from the 3rd to 9th instar stage. The increase in wet weight and prosomal width was isometric throughout the 9 developmental stages. Based on the allometric growth patterns, the juveniles can reach adult size within 4 yr. The results may serve as a basis for the large-scale culture of this species for the purposes of restocking and restoration of natural populations.
KEY WORDS: Horseshoe crab · Tachypleus tridentatus · Juvenile growth · AllometryResale or republication not permitted without written consent of the publisher Aquat Biol 11: 37-46, 2010 Use of captive breeding for population enhancement in animal conservation has increased markedly in the last decade (Olney et al. 1994). In Taiwan, more than 10 000 individuals of second-instar juvenile Tachypleus tridentatus produced artificially in the laboratory were released in the protected area in Kinmen in 2002(Chen et al. 2004), while about 40 000 juveniles were released in the wild in Xiamen, China, in the same year. A successful restocking program relies heavily on an adequate supply of juveniles from artificial breeding, as juveniles released in the wild may suffer high mortality from predation, diseases, food availability, etc. The problems with juvenile horseshoe crabs in culture include low fertilization success, high juvenile mortality and slow growth of the instars (Sekiguchi et al. 1988, Chatterji 1994. The most extensive study of horseshoe crab growth was conducted by Sekiguchi (1988) on the juveniles of Limulus polyphemus and T. tridentatus, which required 9 yr to develop into 14th and 10th instars, respectively. The study itself, however, may not be representative because of both low numbers of specimens and survivors in the study, as an initial number of 44 and 33 ind. were used for L. polyphemus and T. tridentatus, respectively, and only one individual of each species survived at the end of the experiment.The actual life span and exact number of ecdysis events in horseshoe crabs are unknown (Tanacredi 2001, Carmichael et al. 2003, Lee & Morton 2005, as it is difficult to accurately assess age in horseshoe crabs (Botton & Ropes 1988). Goto & Hattori (1929) suggested that Tachypleus tridentatus molts 12 to 13 t...